Bug 989375 - Fix basic layer readback with SkiaGL canvases r=gwright

gfx/layers/CopyableCanvasLayer.cpp

@@ -99,7 +99,13 @@ CopyableCanvasLayer::UpdateTarget(DrawTarget* aDestTarget)
     RefPtr<DataSourceSurface> readSurf;
     RefPtr<SourceSurface> resultSurf;
 
-    SharedSurface_GL* sharedSurf = mGLContext->RequestFrame();
+    SharedSurface_GL* sharedSurf = nullptr;
+    if (mStream) {
+      sharedSurf = SharedSurface_GL::Cast(mStream->SwapConsumer());
+    } else {
+      sharedSurf = mGLContext->RequestFrame();
+    }
+
     if (!sharedSurf) {
       NS_WARNING("Null frame received.");
       return;